An electromagnetic flowmeter carries a diagnostic function of detecting a abnormal state of each part thereof. As for an electromagnetic flowmeter having an exciting circuit to send an exciting current through an exciting coil, it is an especially important to detect abnormalities of the exciting current.
Conventionally, there are a 2-level excitation method, a 3-level excitation method and a remaining magnetism excitation method as an excitation method of electromagnetic flowmeter. The 2-level excitation method includes a positive excitation and a negative excitation by a constant current. As for the 3-level excitation method, a non-excitation is added to the 2-level excitation method (see JP, 2004-354205A, for example). The remaining magnetism excitation method uses an intermittent pulse excitation and realizes low power consumption less than the 3-level excitation method (see JP, 1985-242318A, for example).
Conventionally, as for an abnormality detection of exciting current, the abnormality of exciting circuit is judged by detecting a change of the exciting current with a detector circuit for exciting current provided in the exciting circuit (see JP, 2004-354205A, for example).
Generally, the abnormality detection of exciting current needs a dedicated circuit including a current detection part to detect a current which flows through the exciting circuit, as shown in JP, 2004-354205A. That is, an electromagnetic flowmeter needs composition new for the abnormality detection of exciting current.
An object of the present invention is to provide an electromagnetic flowmeter which can perform abnormality detection of exciting current with sufficient accuracy without adding new composition and a self-diagnosing method of an exciting current circuit unit thereof.